Photochemical Model Performance for PM2.5 Sulfate, Nitrate, Ammonium, and pre-cursor species

SO2, HNO3, and NH3 at Background Monitor Locations in the Central and Eastern United States

Kirk BakerOctober 2006

Lake Michigan Air Directors Consortium

University of Illinois - Chicago

Ammonia Monitor Network

• 9 rural and 1 urban (MI)• Measure SO4, SO2,

NH4, NH3, NO3, HNO3• 24-hr average sample

every 6 days• Samples at most sites

use URG manual• Data collected and

validated for entire year of 2004

Horizontal Grid Domains

Meteorological Domain 36 km Photochemical Domain 36 km

•Vertical atmosphere up to 100 mb (~15 km) is modeled with 16 layers

•Most vertical layer resolution is in the boundary layer

Photochemical Model

• The emissions are based on 2002 emission inventories developed for O3/PM2.5 State Implementation Plans (2004 biogenics)

• Boundary and initial concentrations are monthly averages from a global photochemical model simulation (GEOS-CHEM model)

• MM5 v3.6.X• CAMx 4.30: mechanism 4; CB4+ gas-phase;

ISORROPIA inorganic; RADM aqueous phase• Snow cover; palmer drought index; BELD3

landuse

Model Performance Metrics

• Metrics consistent with EPA modeling guidance:– Bias– Error– Fractional Bias– Fractional Error– r2

• Model performance using daily average measurements: SO2, SO4, NH3, NH4, NO3, HNO3

N

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M

jji

ji

ji

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OP

OP

MNFE

1 1

21

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jji

ji

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OP

OP

MNFB

1 1

21

N

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MNMB

1 1

)(1

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MNMAGE

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Bias

Error

Fractional Bias

Fractional Error

SO2

SO4

NH3

NH4

HNO3

NO3

NH3

PM2.5 NH4

NHX

Fractional Bias (%)

SO2

PM2.5 SO4

Fractional Bias (%)

SO2+ PM2.5 SO4

HNO3

PM2.5 NO3

Fractional Bias (%)

HNO3+ PM2.5 NO3

Bias (ug/m3)

-1

0

1

2

3

4

5

6

NH3 NO3 HNO3 SO2 SO4 NH4

Winter Spring Summer Fall

Fractional Bias (%) by Site

0

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4

6

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14

16

1 2 3 4 5 6 7 8 9 10 11 12

Con

cen

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(u

g/m

3)

AMN CASNET Model AMN Model CASTNET

0

0.5

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1.5

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2.5

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3.5

4

1 2 3 4 5 6 7 8 9 10 11 12

Con

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(u

g/m

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SO2

HNO3

• Monthly average HNO3 and SO2 concentrations at 24 Midwest CASTNET stations (blue dots)

• CASTNET samples are 7 day averages

• No cyclone inlet at CASTNET monitor so concentrations should be a little higher

• CASTNET stations are clustered around the Ohio Valley

•The DON (degree of sulfate neutralization) is estimated to determine if sulfate is completely acidic, totally neutralized, or in-between:

•DON=2 indicates ammonium sulfate; DON=1 indicates ammonium bisulfate; DON=0 indicates particulate sulfuric acid

•Eliminated data where DON>3 and DON<0 for these plots

96/]4[

62/]3[18/]4[

SO

NONHDON

• An indicator of whether PM2.5 nitrate ion formation is limited by the availability of nitric acid or ammonia is the excess ammonia indicator (EA) (Blanchard et al., 2000)

Excess Ammonia (µmoles) = [NH3] + [NH4] – 2*[SO4] – [NO3] – [HNO3]

• When EA<0 then PM2.5 ammonium nitrate formation is ammonia limited

• When EA>0 then PM2.5 ammonium nitrate formation is nitric acid limited

• Model SO4 tends to be fully neutralized much more often than observations

• Model tends to be more ammonia limited than the observations

Model performance for DON (left) and excess ammonia (right)

Remarks

• PM2.5 SO4, NO3, and NH4 ion performance good• Pre-cursor species are well estimated, but not as well as

the PM2.5 ions• Is HNO3 under-measured, over-predicted, or both?• SO2 is systematically over-predicted, but SO4 is well

estimated by the model• NH3 is under-predicted in the winter• Model and observations agree that these sites tend to be

limited by HNO3 in PM2.5 ammonium nitrate formation• Sulfate is usually fully neutralized in the model which

does not always agree well with observation data

Species N RSQ N RSQ

NO3- 487 0.44 463 0.77

SO4= 536 0.49 510 0.71

NH4+ 477 0.35 103 0.72

SO2 558 0.52 42 0.86

Model-Obs Obs-Obs

Model-Observation and co-located observation error (ug/m3) and fractional error (%) metrics

Metric Species Model-Obs1 Model-Obs2 Obs-Obs3

Error NO3- 1.2 1.0 0.50 ± 0.08

Error SO2 4.5 3.39 ± 1.34

Error SO4= 1.5 1.2 0.75 ± 0.11

Error NH4+ 0.9 0.44 ± 0.11

Frac Error NO3- 85.9 84.7 45.0 ± 3.9

Frac Error SO2 96.6 39.0 ± 10.2

Frac Error SO4= 52.1 39.4 33.4 ± 2.7

Frac Error NH4+ 66.8 42.4 ± 8.2

1 model v. ammonia network2 model v. IMPROVE network3 confidence interval based on α=0.05

Wet Deposition (kg/km2) – NADP sites

Fractional Bias Fractional Error

Season N NO3- SO4

= NH4+ NO3

- SO4= NH4

+

Winter 496 -73 -91 -113 92 108 122Spring 535 -27 -9 -50 64 64 81

Summer 493 -14 14 -19 70 74 78Fall 458 -28 -48 -64 68 81 87

SO4= NO3

- NH4+

Quarter 4 Seasonal Average Difference Plot: SO2MOD-BASE

Model predictions with SO2 deposition velocity*2 (red) and without (blue)

SO2 & SO4 performance metrics

Metric Species N BASE SO2 SENS Units

Bias SO2 559 4.0 2.2 ug/m3

Bias SO4= 537 1.0 0.7 ug/m3

Error SO2 559 4.5 3.1 ug/m3

Error SO4= 537 1.5 1.3 ug/m3

Frac Bias SO2 559 90.1 68.1 %

Frac Bias SO4= 537 31.3 23.5 %

Frac Error SO2 559 96.6 80.8 %

Frac Error SO4= 537 52.1 49.0 %

SO2 SO4

Bias by Site (ug/m3)

-2

-1

0

1

2

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4

5

6

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8

IA IL KS KY MI MN1 MN2 MO OH2 OK

NH3 NO3 HNO3 SO2 SO4 NH4